Foam plastic panels are currently very widely employed as insulating panels in refrigerators, refrigerated trucks, picnic coolers and freezers. Refrigerator and freezer manufacturers have been constantly striving to increase the efficiency of their products, and in particular, have attempted over the years to produce a reliable, highly efficient and moderate cost product. In the recent past, attempts to increase efficiency in refrigerators and freezers have achieved success by employing more and better foam insulation in the wall panels of the refrigerators and freezers, by increasing the compressor efficiency and by various design changes, including the relocation of the heat generating components of the refrigerator and/or freezer.
Recently, an added emphasis has been provided to motivate manufacturers to strive for increased efficiency in their products. The Department of Energy and the Environmental Protection Agency have both recently promulgated regulations that will have a pronounced effect upon the design, manufacture and sale of refrigerators and freezers in the near future. The Department of Energy has promulgated regulations mandating substantially increased energy efficiency for home appliances, and, since refrigerators in the United States consume an estimated 7% of the electricity generated in the nation, the proposed regulations mandate a substantial improvement in refrigerator efficiency. The Environmental Protection Agency has promulgated regulations to limit the use of fluorocarbons in all applications, since there is increased concern throughout government, industry and society in general that increased use of fluorocarbons might contribute to depletion of the ozone layer and result in an increase in exposure to ultraviolet rays, which is a significant cause of skin cancer. It is conceivable that, in the not too distant future, the use of fluorocarbons will be totally banned.
Fluorocarbons are used in the manufacture of insulating foam materials such as polyurethane, and as a blowing agent that yields a foam having a high resistance to heat transmission. Fluorocarbons are also utilized in refrigerators and freezers as the working fluid (i.e., refrigerant) circulating between the compressor and the evaporator.
As a result of all of the above mentioned factors, the appliance industry is engaged in extensive research directed towards developing various alterative working fluids for compressors. Possible replacements for common Freon 12 in refrigerator and freezer compressors include Freon 122 and Freon 123. Similarly, the urethane industry is exploring a wide range of alternatives to the use of fluorocarbons for use as blowing agents in foams. However, many of the proposed alternatives to fluorocarbons result in less heat resistance, which leads to poorer insulating properties for the resulting foam. Further, some of the proposed substitute refrigerants have flammability problems. As a result of these factors, the appliance industry is highly desirous of finding solutions to the aforementioned problems that will result in increased appliance efficiency and increased appliance reliability, at moderate to low costs, and without the use of fluorocarbons.
It is well known that excellent thermal insulation capability can be obtained by providing a sealed vacuum between two members. Perhaps the most common device utilizing this principal is the ordinary thermos or vacuum flask. In STANLEY, the assignee's own expired U.S. Pat. No. 1,071,817, patented Sep. 2, 1913, such a vacuum or thermos bottle is disclosed. The STANLEY patent discloses filling of an evacuated space between the inner and outer walls of a bottle or flask with a finely divided material, such as metallic oxides, so as to enable the vacuum to achieve a desired degree of heat insulation at a much reduced gaseous pressure, i.e., with much less exhaustion. While this solution has been quite acceptable for the ordinary thermos bottle, which can be replaced or discarded upon deterioration of the vacuum, in a refrigerator or freezer having an average useful life of twenty or more years, it is essential that, if a vacuum is utilized, the vacuum must be maintained virtually indefinitely without deterioration.
As a result, a great deal of research has been undertaken to attempt to provide a long lasting, highly efficient vacuum insulated panel that could be used in refrigerator and freezer cabinets. These research efforts have led to somewhat diverse solutions to the problems involved in the construction of a long-lasting vacuum insulated panel.
In this regard, various types of vacuums must be defined and distinguished. A "rough" or "soft" vacuum is generally defined as a vacuum having a pressure in the range of 1 to 10.sup.-3 torr (i.e., in the millitorr range). On the other hand, a "high" or "hard" vacuum is generally defined as a vacuum having a pressure in the range of 10.sup.-3 to 10.sup.-6 torr (i.e., in the microtorr range). Thus, a soft vacuum is a vacuum that permits relatively more gases to remain within the evacuated space than in the case of a hard vacuum. Accordingly, while it is substantially easier (i.e., faster and thus less expensive) to obtain a soft vacuum, the thermal insulation efficiency of a hard vacuum is much higher than a soft vacuum. Accordingly, research efforts are now being directed towards developing long-lasting hard vacuum insulating panels.
The difference in thermal conductivity of air or other gases at various pressures can be quite substantial. It is well known that thermal conductivity of air between atmospheric pressure and approximately 10 torr remains relatively constant. Then, there is a sharp drop in thermal conductivity as the pressure is decreased to about 10.sup.-3 torr. There is then little discernable further decrease in thermal conductivity below pressures of about 10.sup.-3 torr. At this level of vacuum, the heat conductivity through the evacuated volumetric space is substantially zero. However, while a hard vacuum is much more efficient as a thermal insulator, a hard vacuum is much harder and much more expensive to obtain, and to retain over the useful life of a refrigerator insulated with vacuum panels.
In addition to the above noted difficulties in forming a hard vacuum insulating panel, other factors are involved in the manufacture and construction of such a vacuum insulating panel. A significant problem exists in that, when evacuating a space to such a low pressure, the walls defining the evacuated space tend to collapse towards each other. This, of course, is highly undesirable, from both a structural as well as from a heat conductive vantage, since, to have a good insulating member, one should avoid direct metal-to-metal contact, and the resulting heat transfer paths that are formed. The most direct previous solution to the problem of panel collapse upon evacuation, has been to increase the structural rigidity of the wall members making up the panel. This, however, has resulted in greater direct conduction of heat through the metal of the wall members and to a decreased insulative value for the vacuum insulated panel as a whole.
As a result, some efforts towards developing vacuum insulated panels have attempted to utilize plastic members to form and/or to support and/or to line the wall elements, because plastic has a generally lower thermal conductivity than metal. However, this has resulted in substantial problems in maintaining the vacuum at a desired insulative effective level over extended periods of time. These problems are caused by the tendency of the plastic materials to slowly release dissolved gases (i.e., outgassing), and by the general permeability of the plastic polymers to the gases of the atmosphere, such as oxygen, nitrogen, carbon dioxide, argon, etc. In order to eliminate these substantial drawbacks to the use of low conductivity plastic materials as all or part of the wall members in the thermal insulated panels, many proposals have been made to either seal the plastic panel or to metallize the surfaces thereof. These solutions have not been entirely successful, and have substantially increased the complexities of manufacturing the panel, and have led to increased manufacturing costs.
In using metal members for the vacuum panel wall members, attempts have been made to use relatively thin members and to provide spacers, reinforcing elements, or supports within the evacuated chamber. The problem with these approaches has been that the supports generally result in areas of increased thermal conductivity through the vacuum, thus resulting in a substantial deterioration of the insulating capacity of the panel as a whole by providing thermal shorts, (i.e., paths of relatively high conductivity) through the panel. Moreover, in forming these internal reinforcing members, substantial problems in the fabrication and uniform spacing of such members have been encountered. As internal reinforcing members, offset dimples, corrugated support panels and sheets containing glass beads, ribs, and rods have been proposed. Each of these methods, however, has required precise alignment of the internal reinforcing members, and has resulted in substantial additional complexities in the fabrication of the vacuum insulated panels as well as in providing additional paths for the conductance of heat through the panels themselves. These additional conductive paths, which have been referred to as thermal shorts, result in a nonuniform heat distribution and lead to substantial deterioration of the heat insulation capacity of the vacuum insulated panel.
On the other hand, the vacuum insulated panels of the present invention, and the method of forming vacuum insulated panels according to the present invention, overcome all of the above mentioned problems and result in vacuum insulated panels of extremely high efficiency, in terms of their impedance to transfer heat, in terms of the long term viability of the vacuum established, and in terms of their manufacturing simplicity; as a result, panels formed in accordance with the present invention result in moderate manufacturing cost.
According to an embodiment of the invention, the entire cavity defined by the metal wall members is filled with a cake or block of powder or particulate material, such as activated carbon black, silica (silica gel), or a combination thereof. A gas-permeable getter-impermeable material is positioned adjacent to a vacuum aperture of the cavity. A vacuum-insulated panel of good durability and insulating qualities is achieved.
According to a further embodiment to the present invention, the filler material contained within the cavity defined by the vacuum panel of the present invention is in the form of a composite material. The composite material comprises a layer of compressed activated carbon and a fiberglass layer. The fiberglass layer is positioned intermediate the carbon layer and the vacuum aperture of the panel. According to this embodiment, the sweeping action of the fiberglass layer into the cavity provides a cleaning action on the weld region and this enables a higher quality weld to be achieved, which results in a better vacuum within the panel and thus a longer-lasting panel.
In accordance with yet a further embodiment of the present invention, the filler material contained within the vacuum panel of the present invention is in the form of a sandwich comprising upper and lower layers of fiberglass material and an intermediate layer of compressed particulate material. The particulate material, which can be compressed in situ, is maintained out of contact with the metal wall members defining the cavity by the fiberglass material. The fiberglass layers act as an insulator and supports the walls of the vacuum panel while the layer of particulate material acts as a getter and as a radiant heat barrier. This embodiment results in extremely long vacuum life and a high insulating value for the vacuum insulated panel.